The present invention relates to a system for automatic gain control and clamp voltage control for a TV (television) receiver.
A band-compressed transmission system called MUSE has been proposed in which a High-Vision (high-definition) video signal for producing images of higher definition or resolution than in the existing standard TV systems is capable of being transmitted by one channel of 27 MHz in bandwidth of a broadcasting satellite.
Such a system is disclosed in "Mono-Channel Transmission System Using Earth Satellite for High-Definition Television (MUSE)" by Yuichi Ninomiya, et al., Television Society Technological Report, System Circuit Research Group, TEBS 95-2, Mar. 22, 1984, Television Society Technological Report Vol. 7, No. 44.
According to the system, which is described in the above-mentioned reference and will not be described here in detail, the signal transmitted is of the type of a component signal composed of a color difference signal time-base compressed and multiplexed with a luminance signal. Further, the signal is of a type provided with a positive polarity of sync signal in order to prevent what is called "synchronization loss" or uneasiness in going into synchronization. For demodulation of the signal, the interpolation of subsampling is necessary using a frame and field memories, which in turn requires the processing of a digital signal. For the purpose, the TV signal is converted into a digital signal by an analog-digital converter (hereinafter referred to as "A/D converter") at the entrance of a decoder. The amplitude and DC potential of the TV signal applied to the A/D converter is thus required to be controlled accurately. Especially, in view of the fact that the TV signal for the MUSE system is comprised of a color signal multiplexed as a component signal, a change in the DC potential makes impossible accurate reproduction of the hue of a demodulated image. In order to prevent this inconvenience, the TV signal of the system is provided with a signal portion so-called the clamp level line representing a reference DC potential level, which is a central level of the signal amplitude. If this signal is converted into a digital signal of 8 bits, for example, the clamp voltage is controlled in such a manner that the potential of the clamp level line is always positioned at the 128th of 256 steps. For this purpose, a clamp level detection circuit is inserted. The 100% amplitude of this signal is always required over the vertical sync signal period called the frame pulse in the video signal. This condition is met by controlling the gain of an amplifier at the entrance of the A/D converter in such a way that the amplitude of the frame pulse is kept always at a predetermined level when the signal was converted into the 8-bit digital signal. This type of gain control is necessary not only for demodulation of signals received in satellite broadcasting but also for compensating for variations in signal amplitude among units when switching other signal sources including VTR and video disk.
As explained above, two types of video signal control are necessary for receiving a television signal image in the above system. For either type, the control circuit processes a signal in analog form while the detection circuit is supplied with a digital signal, so that the interface between the detection circuit and the control circuit naturally requires a digital-analog converter (hereinafter referred to the D/A converter) for converting the digital control signal into an analog control signal. The problems associated with this procedure will be explained with reference to FIG. 2. In FIG. 2, reference numeral 1 designates a TV signal input terminal of MUSE system, numeral 2 a gain control circuit for controlling the amplitude of the TV signal by a control voltage applied externally, numeral 3 a clamp circuit in which the DC potential of the TV signal controlled to a predetermined amplitude by the gain control circuit 2 is controlled by a control voltage applied externally, numeral 4 an A/D converter, and numeral 5 a demodulation circuit for demodulating the original MUSE signal into the corresponding high-definition signal by the digital signal processing. Numeral 6 designates a clamp level detection circuit for digitally detecting the deviation of the DC potential of the clamp level line from the level of 128/256 and generating a clamp level control signal. Numeral 7 designates a D/A converter for converting the clamp level control signal in digital form into an analog voltage. Numeral 8 designates an amplitude detection circuit for detecting the amplitude of a frame pulse set to 100% amplitude, digitally detecting the difference thereof from a maximum amplitude (which is not set to the maximum level of the dynamic range but is generally set with some margin above and under) determined by the demodulation circuit and generating a gain control signal. Numeral 9 designates a D/A converter for converting the gain control signal in digital form into an analog voltage.
In the above-mentioned arrangement, the two control signals are required to have at least eight bits, or preferably, 10 to 12 bits in order to maintain the control accuracy, thereby necessitating two D/A converters of the degree of accuracy for an increased cost. Further, two groups of digital control signals with 8 to 12 bits are required, and the circuit arrangement is inevitably complicated. At the time of channel switching or power-source on switching when the system is out of synchronization, the detection circuit is not expected to operate normally with an abnormal control signal generated, with the result that the amplitude and clamp potential of the TV signal applied to the A/D converter cease to be normal. The pull-in of synchronization is prevented, thereby leading to the problem of a longer time required before a normal image is reproduced.